CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance) is used in IEEE802.11 which is a typical standard for communication used in a wireless LAN.
In CSMA/CA, each wireless communication apparatus transmits a data frame after performing carrier sensing to confirm that a channel to be used is being unused (idle) for more than a predetermined time period.
When a wireless communication apparatus receives the data frame properly, the wireless communication apparatus returns a receipt confirmation frame after a predetermined period (an SIFS (Short Inter Frame Space)) to notify the data frame sender of the normal receipt of the data frame. The receipt confirmation frame may be an ACK frame, a Block ACK frame, or a CTS frame. The wireless communication apparatus on the receiver side answers with the receipt confirmation frame which varies according to the type of the received data frame.
SDM (Space Division Multiplexing) is known as one of wireless communication systems. In SDM, a wireless communication apparatus on the sender side forms directional radio waves by using an adaptive array antenna or the like and transmits (spatially multiplexes) different radio waves to a plurality of wireless communication apparatuses simultaneously and with one and the same frequency. That is, in SDM, a wireless communication apparatus on the sender side (e.g. an access point device) can transmit different data frames to a plurality of wireless terminals respectively, simultaneously and with one and the same frequency.
When SDM is used in an IEEE802.11 standard-based wireless LAN system, there is however a possibility of occurrence of a situation where the access point device may receive a receipt confirmation frame from another wireless terminal while the access point device is transmitting a data frame to one wireless terminal. This situation is referred to as a “collision.”
The situation how the collision occurs will be described below.
First, the access point device starts transmission of different data frames to a plurality of wireless terminals simultaneously and with one and the same frequency by SDM. When the size of a data frame to be transmitted to one wireless terminal STA1 is large compared with the sizes of data frames to be transmitted to other wireless terminals, the access point device may terminate transmission of the data frames to the other wireless terminals than the wireless terminal STA1 while the access point device transmits the data frame to the wireless terminal STA1. Because the other wireless terminals than the wireless terminal STA1 return receipt confirmation frames, there is a possibility of occurrence of a situation (collision) that the access point device may receive the receipt confirmation frames from the other wireless terminals than the wireless terminal STA1 though the access point device is transmitting the data frame to the ratio terminal STA1. As described above, when SDM is used in an IEEE802.11 standard-based wireless LAN system, there is a problem that the access point device cannot receive any receipt confirmation frame.
As a method to solve this problem, there has been disclosed a technique in which the return start timings of receipt confirmation frames from a plurality of wireless terminals are synchronized when different data frames are transmitted to the plurality of wireless terminals simultaneously and with one and the same frequency by SDM. An example of such technique is disclosed in the following document (1).    US 2005/0147023 A1 (1)
In the technique disclosed in the document (1), occurrence of collision can be prevented even when SDM is employed in a wireless LAN system compliant with the IEEE802.11 standard.
As shown in FIG. 9, the IEEE802.11e standard which is an enhanced IEEE802.11 MAC layer protocol allows data frames to be transmitted continuously on an exclusively used channel during a period called TXOP (Transmission Opportunity) after the authorization of data frame transmission is acquired once.
During the TXOP period, any number of data frames is allowed to be transmitted continuously at intervals of SIFS, as described in the following document (2).    “Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) specifications, Medium Access Control (MAC) Quality of Service (QoS) Enhancements”, IEEE Std. 802.11e-2005 (2)
In the technique described in the document (2), because it is unnecessary to acquire the authorization of data frame transmission again whenever a data frame is transmitted, the overhead (AIFS (Arbitration Short Inter Frame Space) and backoff period) required for acquiring the authorization of data frame transmission can be reduced to achieve improvement in throughput and system capacity.
As described above, in the technique disclosed in the document (1), even when SDM is employed in the IEEE802.11 standard, occurrence of collision can be suppressed by synchronizing the timings of receipt confirmation frames returned from the respective wireless terminals.
However, when SDM is employed in the technique disclosed in the document (2), that is, in the technique in which data frames are transmitted continuously during the period of TXOP defined in IEEE802.11e, the following problem occurs.
Assume now that an access point device transmits data frames continuously to a plurality of wireless terminals simultaneously and with one and the same frequency during the TXOP period. The access point device transmits next data frames without carrier sensing again when an SIFS has passed after reception of receipt confirmation frames as replies to the transmitted data frames from the plurality of wireless terminals.
The receipt confirmation frames are various in terms of type and size. The transfer rate in transmission of a receipt confirmation frame to the access point device varies according to each wireless terminal. For this reason, even when the timings of returning receipt confirmation frames from the plurality of wireless terminals are synchronized by the technique disclosed in the document (1), the time required for completion of the access point device's reception of a receipt confirmation frame returned from each wireless terminal (the time required for delivery of a frame (hereinafter referred to as frame time length)) varies.
Although the access point device transmits a next data frame when the SIFS has passed after reception of a receipt conformation frame, the time point of transmission start of the next data frame varies because the time required for completion of reception of the receipt conformation frame varies.
For this reason, when the time length difference between a receipt confirmation frame from the wireless terminal STA1 and a receipt confirmation frame from the wireless terminal STA2 exceeds the SIFS as shown in FIG. 10A, transmission of a next data frame to the wireless terminal STA1 is started though reception of the receipt confirmation frame the wireless terminal STA2 is not completed. There is a problem that collision occurs.
On the other hand, if transmission of a next data frame is intended to be started when a predetermined time (SIFS) has passed after completion of reception of the latest receipt confirmation frame as shown in FIG. 10B, the channel seems free for the SIFS to the wireless terminal (the wireless terminal STA2 in FIG. 10B) returning the receipt confirmation frame latest in terms of reception in the access point device but the channel seems free for a period longer than the SIFS to the other wireless terminal (the wireless terminal STA1 in FIG. 10B) than the wireless terminal returning the receipt confirmation frame latest in terms of reception in the access point device. For this reason, there is a possibility that the other wireless terminal than the wireless terminal returning the receipt confirmation frame latest in terms of reception in the access point device will acquire the authorization of transmission even when the access point device is continuously transmitting data frames.
In such a case, data frames transmitted continuously by the access point device collide with a data frame transmitted by the wireless terminal which acquires the authorization of transmission because the channel is free for a period longer than the SIFS. There is hence a problem that the access point device cannot transmit data frames continuously at intervals of the SIFS during the TXOP period.
As described above, in the IEEE802.11-based wireless LAN system, when the access point device transmits spatially multiplexed data frames to a plurality of wireless terminals and transmits such data frames continuously at intervals of the SIFS during the TXOP period, there is a problem that collision will occur because the transmission timing of a data frame to one wireless terminal overlaps with the reception timing of a receipt confirmation frame from any other wireless terminal.
For this reason, there is a problem that MAC efficiency cannot be improved because the concept of TXOP (the technique for transmitting data frames continuously at intervals of the SIFS during the TXOP period) cannot be applied to the case where SDM is used in the IEEE802.11-based wireless LAN system for spatially multiplexing and transmitting data frames.